Microstructure and Thermal Degradation of PVC with Improved Thermal Stability

Doctoral thesis, 1994

Although the low thermal stability of poly(vinyl chloride) (PVC) has been an area of intensive research, the relation between microstructure and degradation behaviour is still not fully understood. This thesis deals with the structure, thermal degradation and modification of ordinary PVC. Samples with various microstructures were obtained by varying the polymerisation conditions. Commercial polymers obtained at different polymerisation temperatures and a polymer obtained with an ionic initiator were also studied. In addition, two types of polymerisation initiators, perester and peroxy- dicarbonate, were compared. PVC was treated with ethanol, trimethylaluminium and dibutyltin maleate in order to substitute labile defects with more strongly bonded groups. The samples were characterised and the degradation behaviour was studied, both in an inert atmosphere and in the presence of HCl. It was found that the content of branches and double bonds in the polymer markedly depends on the polymerisation temperature and the conversion. Methyl branch is the most frequent defect, while butyl branch is the most frequent structure connected with thermally labile chlorine. The formation of the latter is favoured at high polymerisation temperature and at high monomer conversion. It was shown that the amount of butyl branches can be reduced by using a semi-batch polymerisation process. Degradation experiments revealed that there is a strong correlation between the concentration of labile chlorines, calculated as the sum of internal allylic and tertiary chlorines, and the dehydrochlorination rate. PVC obtained at low polymerisation temperature and modified PVC exhibited the same degradation behaviour. In addition to showing a low dehydrochlorination rate, the polyenes became short. The results can be explained by HCl catalysis of the polyene propagation. A low dehydrochlorination rate leads to short polyenes due to a relatively low HCl concentration in the material during degradation. From degradation experiments it was also found that the polymerisation initiator has a significant effect on the early discolouration. It is concluded that PVC becomes early discoloured when remaining initiator decomposes and the radicals then formed initiate dehydrochlorination. The best thermal stability was obtained for PVC treated with trimethylaluminium. The degradation rate was reduced to less than 20% of that of unreacted PVC. Characterisation of the sample showed that labile chlorines had been removed to a large extent. This is most probably the main reason for the excellent thermal stability of alkylated PVC, but traces of aluminium chloride may also contribute. It is suggested that low concentration of homogeneously dispersed aluminium chloride catalyses the reverse dehydrochlorination reaction.